FAQs

What is Spectrally Controlled Interferometry (SCI)?

A

SCI is a new source modality for interferometry. The previous modalities are broadband sources and lasers. Broadband are hard to set up but isolate the surface under test. Lasers are easy to setup but produce fringes everywhere, degrading measurement accuracy. SCI is easy to set up and isolates the surface under test for optimal accuracy.

When was SCI discovered?

A

The principle of SCI was first published in 1997 by Professor Schwider (“White Light Fizeau Interferometer”, 1 March 1997 Vol. 36, No. 7 APPLIED OPTICS). The barrier to commercialization was creating a practical, yet flexible SCI source. Dr. Olszak, one of ÄPRE’s founders, developed the architecture for a practical source, and ÄPRE developed the product based on this SCI architecture.

Can SCI be applied to any interferometer?

A

All ÄPRE S-Series interferometers are SCI ready, so they can be upgraded in the future or shipped with an SCI source.

Why is it easy to align a part with SCI?

A

SCI has an alignment mode which has high temporal coherence, like a laser. In this mode the laser Fizeau interferometer operates just like a standard interferometer we are all familiar with.

What range of interference cavities does SCI operate over?

A

The range is determined by the minimum isolation cavity and longest cavity with ample contrast fringes for data acquisition. Cavity length is defined as Optical Path Length or OPL = n (index of refraction) * l (physical distance between the two surfaces of interest).

The longest cavity OPL SCI can measure is ~500 mm as of November 2018.

How fast can SCI shift between cavity lengths?

A

SCI is controlled electronically. Cavity length shifts can occur at the camera frame rate. Therefore in production settings, identical parts with multiple cavities can be measured in a second, where the previous multiple setups and measurements, if possible, could take up to 15 minutes for each part.

Are there limitations to SCI phase data acquisition?

A

No! This is an exciting aspect of SCI. The phase shifts are controlled by the source spectrum phase, not wavelength. There is no limitation for SCI regarding cavity length from the smallest 50 μm to the maximum OPL the future brings. When wavelength is controlled to phase shift, instead of spectrum phase, a 633 nm interferometer is limited to about 9 mm minimum OPL and approximately 2 meters maximum OPL. A severe limitation both for separating surfaces AND the ability to phase shift.

Any other limitations?
When two cavities have the same OPL to within 75 μm of each other, fringes will be produced in both cavities simultaneously hindering measurement. Simply shifting one cavity slightly will isolate the two cavities. So this is rarely an issue.

What wavelength does SCI operate at?

A

The nominal wavelength is 660±2 nm. Other wavelengths are possible.

Can SCI data acquisition be Vibration Tolerant?

A

Yes. ÄPRE’s Vibration Tolerant PSI is applied directly with the same benefits.

Can SCI measure distances?

A

SCI positions the fringes on the cavity of interest by controlling the source spectrum. Research has been performed on the measurement of spherical surface radius of curvature and optical thickness. Both have been initially demonstrated in the laboratory and are an area of ÄPRE research. See Chase and Artur’s OptiFab paper, “Spectrally controlled interferometry for measurements of flat and spherical optics” (open “Technical Papers” on the linked “Resources” page) for early results.

What measurement are now possible with SCI that were not previously possible?

A

Measurements that were limited by back reflections are now possible with SCI. An example list is: